The Heat of Formation for a compound is the heat generated when elements combine to form the compound. A positive Heat of Formation is otherwise known as an exothermic reaction, and these by far are the most numerous, as reactions tend towards the more stable, and less energetic compounds, the energy given off as heat. So as soon as combination occurs, heat is released. The other process, a negative Heat of Formation, or endothermic reaction, happens less often, as these reactions tend towards the creation of less stable and more energetic compounds, absorbing energy during the reaction in the form of heat. These are slower, as it is the equivalent of "running uphill" to add energy, or absorb heat to create the compound, and the addition of energy makes it harder for the elements to react with each other, because the more energetic they are, the less likely they are to combine. The products of endothermic reactions are almost always unstable, and some can break down to their components with explosive violence.